The Ice Fracture Explorer

Europa, the second Galilean moon of Jupiter, has been my favorite planetary body for a long time. The reason I like Europa so much is that it’s a world whose orbital dynamics with Jupiter, its orbital resonances with the other Galilean moons, and its own rigid-body dynamics have a strong hand in creating its surface features – and giving it the potential to harbor life. It’s one of perhaps two or three extraterrestrial places in the Solar System where we might hope to find life. Europa is also easier to get to than Enceladus or Titan. As such, I think it ought to be one of the highest-priority exploration targets for robotic space probes. (Human exploration would be nice, too, but if you think radiation exposure on the way to Mars is hard, you don’t even want to consider putting people in the Jovian system!)

Thanks to magnetometer measurements and images from the Galileo mission, it’s pretty much established at this point that Europa has an icy outer shell over a global liquid ocean, with a rocky core on the inside.* The only question is how thick that ice shell is – I’ve read estimates ranging from 10 meters to 100 kilometers, with a pretty high confidence of ones to tens of kilometers. The ice shell gives rise to a number of interesting surface features. A particularly cool sort of feature, found with global extent across Europa, is the double ridge.

A prominent double-ridge feature on Europa, most likely a crack in the icy shell

Planetary scientists have a number of models for how these double ridges form, and they generally seem to agree that the ridges mark the locations of cracks in the ice crust. One especially well-established model suggests that these cracks occur when Jupiter raises tides in Europa’s ocean – just like how the Moon raises tides in terrestrial oceans, but much stronger, because Jupiter is frakking huge compared to Earth’s moon. Europa’s ice crust bulges out over the ocean’s tidal swell and then cracks under the incredible stress. (I like to take a moment to think about the mindbogglingness of that statement: the whole moon’s surface cracks. I’ve stood on a frozen pond when a crack pings through the foot or so of ice on top of the water – Just imagine standing on Europa when this happens!) Once a crack forms, the tides don’t go away. As Europa rotates, about once every three and a half Earth days, the tides periodically lever these cracks apart and squeeze them back together again. In this model, every time the cracks gape open the subsurface ocean gets exposed to space. The surface water boils and rapidly crusts over with ice, and when the cracks get smushed closed, all this ice gets crushed up and forced to the top and bottom of the crack, forming the ridges. The ridges appear in pairs because the crack opens up again after that. These double-ridge features are mounds of crushed ice flanking passages into Europa’s ocean!†

Dr. Richard Greenberg is a planetary scientist who thinks that these cracks in the ice shell might be potential sites for life to take hold. Unlike the rest of the subsurface ocean, they get exposed to sunlight, which means that photosynthesis could take place. The periodic in-and-out forcing of the crack would also drive strong currents, which is another energy source Europan life could use. (Those aren’t the only energy sources: other possibilities include thermal gradients in the water, volcanic vents on the ocean floor, or even induction as Europa travels through the Jovian magnetic field.) Of course, that life would also have to adapt to the crack opening and closing once every 3 1/2 Earth days!‡

We do at least know, from the Galileo mission, that these cracks often have accompanying veneers of organic (e.g. carbon-based) molecules and salts splashed onto the ice surface. This is why the cracks appear as brown stripes in large-scale context images. The crack/veneer combination suggests that there are organic molecules and salts in the Europan ocean, and that those compounds get pumped to the surface through these cracks.

So, let’s take stock: Europa is the only extraterrestrial world with a global liquid water ocean, there is a definite possibility for life in that ocean, and these double-ridged cracks are a possible gateway into the alien biosphere.

Well, then, let’s go diving! Read on for my concept system architecture for an ambitious Europan ocean-exploring mission, which I call the Ice Fracture Explorer.

The Ice Fracture Explorer, or IFE, would be a combination lander/penetrator vehicle that I imagine to be a little smaller than the size of one of the MER rovers. Ideally, several IFEs would accompany an orbiter to Europa. The orbiter component of the mission would contain instruments designed to give the planetary scientists on the mission enough information to select a few double-ridged cracks that are actively being worked open and shut by tides. The flight controllers would then dispatch an IFE to each of those cracks.

Soft landing on a double ridge interior

The IFE includes thrusters and landing pads to make a soft landing on the surface. Europa’s surface gravity is less than half of Mars’, so a soft landing should be easier than the successful Viking or Pheonix missions, even without the benefit of a parachute. The IFE landers would target the inside-facing slopes of the double ridges, as shown above. Each vehicle also includes cameras and instruments, and the IFE will pause on the inside of the double ridges to relay data back to Earth.

The goal of these images are to verify that the crack is opening and closing each tidal cycle, and to establish the timing of the cycle. Once the cycle is known, flight controllers will uplink commands to the IFE to begin the second phase of the mission.

The IFE will wait until the crack is closed, and then separate form the landing legs and inflate some gas-bladder cushions, causing the vehicle to roll down towards the center of the double ridge. Using its thrusters for attitude adjustment, the IFE will right itself, centered over the crack.

Bouncing on air bags

At this point, the gas cushions would deflate. Since the IFE has to operate in the middle of the double ridge, I imagine it will need to run off an RTG power supply instead of solar panels.

After righting itself, the spacecraft deflates the air bags

Next, the IFE would fire projectiles into the crushed-ice ridges on either side of the vehicle. These projectiles could be barbed, contain chemical flash heaters, or anything else design to make them really stick into the ice, because they would be the anchors for twin tether lines that unreel from the spacecraft. The IFE would also deploy a high-gain antenna for communicating with the orbiter overhead, since the mission will have to happen very quickly from this point on.

Anchored to the walls, and antenna deployed

Now, the spacecraft waits. Inside the IFE, the tethers would be mounted so that they unreel in unison. When the ice crack opens up again, the tether cables will support the IFE above the center of the crack.

The mechanism pays out both cables at the same rate

As Jupiter rises overhead wobbles around zenith, its tides will pull apart the two sides of the ice fracture. The IFE will be suspended in the middle as the crack opens, with nothing below it until the ocean 1-10 km down! At this point, the IFE will drop its deflated cushions and begin to deploy a smaller penetrator vehicle from its underside. The penetrator is a small, two-stage vehicle with two instrument packages, a hard-shell body, and a data line connecting it to the IFE’s main bus.

Hanging over the abyss

The IFE would drop the penetrator, letting gravity take it down into the fracture. As it falls, the penetrator will photograph the fracture walls in visible and infrared light. Data will be rapidly carried up the line to the main bus hanging overhead, which will relay the images to the orbiter

Eventually, the penetrator would hit the ocean surface. The water would have iced over, but the weighted penetrator with its reinforced lower body would smash through the ice and reach the liquid water below. At that point, a buoyant surface instrument package would separate from the lower penetrator, which would continue down into the water. The surface instruments would try to identify any interesting chemistry or biology occurring at the water surface, where photosynthesis might take place. The lower body of the penetrator would simply try to go as far down as it can, illuminating the depths and taking pictures.

The IFE drops two instrument packages, one that stops just under the surface and one that goes as far down as possible

The IFE will have perhaps less than one quarter of a Europa day to operate. When the crack closes, it will be crushed inside unless something exceptionally lucky happens or the ice shell turns out to be very, very thin. One possible mission design might call for the IFE main bus to reel in the tethers when the crack closes in an attempt to survive for a second day, but the penetrators will likely be lost.

If the Europan ice shell is indeed hundreds of kilometers thick, then a data line might not be the best solution for penetrator-to-main-bus communication. I don’t think radio or laser communication would work well across the water/vacuum interface, though. In that case, one possible mission profile might be to wait for the crack to start closing, when the water level inside the crack might be squeezed up towards the surface. However, there would likely be much less data available.

Finally, since the IFE would be crushed inside the fracture as it closes, it will have to relay all its data to the orbiter before it loses communications capability. The orbiter would have to be timed to be overhead for the IFE to relay all its data – perhaps in an eccentric orbit that spends the entire quarter-day over the crack, or maybe just coming into communications range of the IFE during its last moments, so the IFE can dump its entire data store and then beam images out as it can before it gets squished.

All this discussion highlights one fact: Europa is a unique challenge. Though it is one of the very few places in the Solar System that we can imagine harboring life, the mission design to explore the Europan biosphere is very difficult and requires many stretches of space technology. The requirements analysis and detailed design of such a mission would take a great deal of innovation and effort. Those are challenges that I would love to see the space program address, though – because the discovery of extraterrestrial life would have a profound impact on our science and society. Diving into the cracks in Europa’s icy shell may be our best bet at making such a discovery!

15 Responses to The Ice Fracture Explorer

I love the mission profile, and indeed the proposal for life existing within these fracture zones. I was initially a bit doubtful of that reading this, and I still wonder whether photosynthesis would be the primary niche for energy capturing life in a possible Europa biosphere.

The fact that the cracks only open for the tidal period isn’t so much of an issue for me, photosynthesizing life on Earth has to deal with periodic lack of access to the sun every night. Europan life would be facing a similar challenge, but with a longer period of darkness (also a longer period of being in the light, if I understand this correctly). I would be curious to see a model of how Europa’s tidal system with Jupiter compares with where Europa’s solar exposure is. I find it doubtful that every period these cracks are forced open that they are also facing the sun. This could provide some unique challenges, but again, not insurmountable for life on the face of it.

What I find harder for life to deal with, especially combined with the above challenges, is the much lower amount of solar energy reaching Europa. We know from the challenge of powering our own probes that we send to Jupiter that it is very hard to gather enough light out there to power anything. I think the new Juno probe is going to be our first to attempt solar power at that distance. Again, this isn’t to say that I don’t think life could find a way to survive this way, life is fantastic at breaking through any preconceptions we make about what conditions will be too challenging for it. I do, however, think that life on Europa is more likely to be driven by hydrogen sulfide chemosynthesis or other volcanic compounds as the primary source of energy driving the ecosystem, with photosynthesis being more of a secondary niche. I suppose a reverse of what happens on Earth.

That doesn’t make these ridges a bad place to look for life, of course. First, they’re the easiest place to try and find life anywhere on the moon. Second, even if that location isn’t the driver for the Europan ecosystem, if we’ve learned one thing from life on Earth it is that if life can find a way to exist somewhere, it probably will exist in that somewhere.

Now, as for your data transmission problem to the orbiter, I had an idea on that. If the problem is one of the lander getting destroyed when the ridge closes, why not leave something outside of the ridge? The lander is landing outside of the ridge and then rolling into it. Before that, leave a sublander behind that essentially amounts to a data storage device, high gain transmitter, and a power source large enough to support both. When the primary lander in the ridge has collected its data, it can send it either by a cable or RF transmission (if that’s possible, I suppose there would be line of sight issues), with the sublander safely storing the data outside of the ridge for transmission back to the orbiter.

That might be adding an extra layer of complexity and more failure points that you don’t want in a probe, but it could also simplify the data recovery problem.

True, that would be a good approach to solving the data recovery issue! I think the problem then becomes how large the mission’s mass budget is. I suppose one way to do it would be to have the main lander sit on one side of the ridge, shoot the cable to the other ridge, and reel out the penetrator to the middle instead of suspending the whole shebang above the crack. The advantage of keeping the whole package centered is the opportunity to take lots of top-down images of the fracture.

Europa is in nearly synchronous rotation around Jupiter. (1 Europan day ~ 1 Europan year). These models for tidal forcing suggest to me that the cracks would spend ~50% of the time “open,” so each crack would get around 1.75 Earth days with the ocean exposed to the surface every 3.5 Earth days. However, which cracks are “active” varies from time to time, so we are indeed talking about a very, very small fraction of the Europan surface. Regardless of its suitability for photosynthesis, those crack are probably the best, and most predictable, surface access to the ocean beneath.

You should really check out the literature on Europa’s biosphere. I’ve seen some pretty cool proposals: one of the neatest ideas for a Europan plant-analog is an organism that consists of long filaments. As the filaments get dragged through Jupiter’s magnetic field, the field induces a current along the “plant” so that it generates energy. With that (or tidal energy, or volcanic energy, …) as the substitute for photosynthesis, you could grow a whole biosphere without sunlight!

Correct me if I’m wrong, but doesn’t Europa’s rotation being tidally-locked to its orbital period mean that Jupiter always stays in one place in the sky for any given position on Europa’s surface? That doesn’t prevent tidal effects, but it might mean that they wouldn’t be timed to the rotation. As I understand it, tidal stresses are coupled from Jupiter to Europa through Io and Ganymede, which pump Europa’s orbit into a constant, though small, eccentricity.

Right you are, on both counts! Europa is tidally locked to Jupiter, meaning that Europa’s rotation period is very close to Jupiter’s orbital period. If Europa’s orbit was circular, that would result in Jupiter maintaining the same position in the Europan sky.

However, Europa’s orbit does have a small eccentricity, because it is in a resonance with the orbits of Io and Ganymede that prevents its eccentricity from damping all the way to zero. That slight eccentricity means that an observer on Europa looking up at Jupiter will see it slowly weave back and forth in the sky. So, the tidal bulge of the oceans also moves back and forth under Europa’s ice crust. It’s this “tidal working” that is responsible for opening and closing the cracks – and for forming them in the first place!

There’s also considerable evidence that Europa’s rotation is just slightly asynchronous, meaning that over a very long time Jupiter moves across the Europan sky. That’s how Europa ended up with cracks covering its entire surface, instead of just on bits pointing directly towards and away from Jupiter.

I will admit that I played a bit fast and loose with my language, for the sake of imagery. “As Jupiter rises overhead” sounded a bit more poetic to me than something like “as Jupiter wobbles around the zenith!”

Do we have evidence these cracks open and stay open in that fashion? That’s a LOT of ice to stay stably open, especially in a repeatable fashion. I’d expect that things would crack at different points, or at least that no specific point would be reliably open under any given cycle (just as you get earthquakes on a fault line, but you can’t reliably say which PART of the fault will actually give way)

My initial guess when looking over these things was that if a crack was to open in Europa’s surface, there would be an immediate explosion of water vapor as the water (which is liquid and at, roughly, freezing point) was exposed to vacuum, effectively boiling up out of the planet towards the surface. At some point the mixture might blast out to the surface, or, more likely, will deposit on the sides and seal up the crack lower down, then get squeezed out when it closes.

The question is actually very relevant to me as the last book I released (Threshold, sequel to Boundary, with my coauthor Eric Flint) left Our Heroes stranded on Europa, and I plan to do a lot with them on Europa for the sequel.

Good questions – and certainly, this plan rides on that model for “tidal working” of the cracks being accurate.

My understanding of the model is that the surface starts off solid until a crack begins from some weak point in the ice. As the tidal bulge swells, the stresses in the ice shell increase and the crack propagates forward from that point like a zipper unzipping. This model is borne out by evidence – many cracks on Europa have curved, or “cycloid,” shapes, which is the shape that follows the direction of maximum tidal stress as it evolves over the course of each Europan day. So, that shape suggests that the cracks form over the course of many Europan days by propagating along from some starting point.

That’s how cracks *first* open up. Once open, though, the crack is a weak point in the ice and the periodic actions of tides can more easily force them to open up again and again. I’m not sure exactly how the fractures would open on those subsequent cycles. That’s one reason why I suggested landing the probe on the inner surface of a double-ridge first – so that we could observe the fracture and try to characterize how and when it opens before rolling the probe in to deploy the penetrators.

I don’t think I’d expect any explosive geysers. The exposure of water might be more gentle as the fractures widen and, after all, this water is likely not under pressure like the liquid pockets that give Enceladus it’s geysers. Still, these fractures are probably the easiest access points from the surface to the ocean on Europa.

Interesting. The water should be under some level of pressure though — just as magma under the Earth’s crust is under pressure. There’s lots of ice above. Plus exposing water to vacuum, I would expect, will cause boiling right away — an effect which will be like popping the top on a shaken pop-bottle.

In any case, now I have to wonder about the tectonic environment. If I’m sitting on Europa, am I going to be feeling Europaquakes frequently, and how violent will they be? Are there areas that will be more or less quakey (e.g., the Conamara Chaos)?

Off the top of my head, from the shape of the tidal bulge, the sub- and anti-Jupiter points would have more of this ice-cracking activity. The jury is still out on how chaoses form – my favorite theory is that they are impact features – so I’m not sure how quakey they might be.

What’s so crucial about getting the top down images of the fracture? The whole idea of spanning the gap suspended on cables seems to depend on a lot of things going exactly right. Why not just anchor the lander on one ridge and drop the penetrator over the side? If there are plants clinging to the edges as suggested that would put the probes right in the middle of them.

It’s not crucial, but those images would be nice to have. More important, in my opinion, is dropping the probe down the middle of the fracture – so that it doesn’t get hung up on ice protrusions or those plants. Certainly, one potentially easier way to do it might be to land on one side of the double ridge and shoot a single cable across to the other side, then reel the penetrator out to the middle.

While all this depends on getting a lot of things right, I don’t think it’s any more complex than, say, the MSL skycrane landing system. I do advocate sending more than one IFE lander with the same mission to mitigate the risks!